Multiple applications for digital video communication and storage currently exist, and multiple international standards have been and are continuing to be developed in response. Low bit rate communications, such as video telephony and videoconferencing, led the International Telecommunication Union (ITU) to develop the H.261 standard with bit rates as multiples of 64 kbps and the Motion Picture Experts Group (MPEG) to develop the MPEG-1 standard, which provides picture quality comparable to that of Video Home System (VHS) videotape.
ITU-T H.264/MPEG-4 is a recent video coding standard that makes use of several advanced video coding tools to provide better compression performance than existing video coding standards such as MPEG-2, MPEG-4, and ITU H.263. The core of all of these standards is a hybrid video coding technique of block motion compensation plus transform coding.
Generally, block motion compensation is used to remove temporal redundancy between successive images (frames), whereas transform (e.g., discrete cosine transformation, or DCT) coding is used to remove spatial redundancy within each frame. As will be illustrated below, H.264/AVC functions include quantization of the transforms of block prediction errors (either from block motion compensation or from intra-frame prediction and based upon macroblocks, or MBs, of 16 pixels by 16 pixels). The quantized transform coefficients are entropy encoded. A rate control unit is responsible for generating a quantization parameter (qp) by adapting to a target transmission bit rate and output buffer-fullness; a larger quantization step implies more vanishing and/or smaller quantized transform coefficients which means fewer and/or shorter codewords and consequent smaller bit rates and files. The entropy coding for the quantized transform coefficients may be either context-adaptive variable-length coding (CAVLC) or context-based adaptive binary arithmetic coding (CABAC).
More generally, after quantizing, the computed signals are converted to several meaningful symbols, and each symbol is encoded with some entropy coding technology. Because the essence of entropy coding is to assign shorter codewords to more frequent symbols and longer codewords to less frequent symbols, entropy coding modules typically employ lookup tables of variable-length coding (VLC) codewords (often called “VLC tables”) to encode symbols efficiently.
As video coding standards expand, they have begun to encounter advanced applications that would particularly benefit from higher compression efficiency. To achieve higher compression efficiency, recent video coding standards have made several modifications to previous entropy coding techniques. The idea common among these modifications is adaptive switching of the coding parameters so that the size of the generated coefficients may be minimized. Typical categories of techniques used in recent video coding standards are:                (1) Coding as side information: Coding parameters (VLC table indexes in most cases) are encoded as side information. A decoder decodes the side information to use the designated coding parameters.        (2) Computing from surrounding information: Based on the recognition that spatially neighboring coefficients tend to have some similarity, coding parameters are computed from information regarding surrounding coefficients.        (3) Using accumulated statistics: An initial set of coding parameters is generated and thereafter updated based on the coded symbols.        
Category (3) includes the adaptive VLC scheme used in scalable video coding (SVC), which is currently being standardized as Annex G of H.264/AVC. In a wider sense, the arithmetic coding technology used in the Joint Photographic Experts Group (JPEG) 2000 standard and the H.264/AVC standard are classified in category (3) as well, because the context of the arithmetic coder is updated depending on the coded binary symbols. In this way, the entropy coding technique using accumulated statistics (hereinafter denoted as “accumulated statistics-based entropy coding”) is used in various coding standards.
Though they are effective, the computational intensity of the CAVLC of H.264/AVC and the adaptive VLC of SVC (Annex G of H.264/AVC) and other such methods presents a problem, especially for mobile, battery-powered devices such as digital cameras.